<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* 
            给一个宽，不给高，高是边框撑开的
            左右两边的多余出来的部分是用透明色撑开的
            边框的宽用来控制四边形的大小
        
        */
        .box {
            border-bottom: 100px solid red;
            border-left: 50px solid transparent;
            border-right: 50px solid transparent;
            height: 0;
            width: 100px;
        }
    </style>
</head>

<body>
    <div class="box">

    </div>
</body>

</html>